This disclosure relates to electrochemical cells, and, more particularly, to an apparatus and methods for improving cell operation.
Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, a section of an anode feed electrolysis cell of the related art is shown at 10 and is hereinafter referred to as xe2x80x9ccell 10.xe2x80x9d Reactant water 12 is fed to cell 10 at an oxygen electrode (e.g., an anode) 14 where a chemical reaction occurs to form oxygen gas 16, electrons, and hydrogen ions (protons). The chemical reaction is facilitated by the positive terminal of a power source 18 connected to anode 14 and a negative terminal of power source 18 connected to a hydrogen electrode (e.g., a cathode) 20. Oxygen gas 16 and a first portion 22 of the water are discharged from cell 10, while the protons and a second portion 24 of the water migrate across a proton exchange membrane 26 to cathode 20. At cathode 20, hydrogen gas 28 is formed and is removed for use as a fuel or a process gas. Second portion 24 of water, which is entrained with hydrogen gas, is also removed from cathode 20.
Another type of water electrolysis cell that utilizes the same configuration as is shown in FIG. 1 is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell also utilizes the same general configuration as is shown in FIG. 1. Hydrogen gas is introduced to the hydrogen electrode (the anode in the fuel cell), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in the fuel cell). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, a hydrocarbon, methanol, an electrolysis cell, or any other source that supplies hydrogen at a purity level suitable for fuel cell operation. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water.
Conventional electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a membrane electrode assembly (hereinafter xe2x80x9cMEAxe2x80x9d) defined by the cathode, the proton exchange membrane, and the anode. Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by flow field support members such as screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA. Because a differential pressure often exists across the MEA during operation of the cell, pressure pads or other compression means are employed to maintain uniform compression of the cell components, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
Referring now to FIG. 2, a conventional electrochemical cell system illustrating the spatial relationship between the active area (defined by the electrodes and the space therebetween) and cell frames is shown at 20. In cell system 20, the MEA 22 is typically supported by the flow field support members 24 and clamped between cell frames 26. Limitations inherent in the precision manufacture of flow field support members 24 and cell frames 26 result in the presence of a first gap 30 of dimension l1 between a peripheral outer surface of flow field support member 24 and an inner boundary surface of cell frame 26 during the assembly of the cell. When the cell is fully assembled and MEA 22 is supported within cell frames 26, the pressure differential is such that the pressure on one side of MEA 22 is higher than the pressure on the other side of MEA 22. During operation of the cell, MEA 22 must be capable of supporting this pressure differential. First gap 30 between cell frame 26 and flow field support member 24 oftentimes exceeds a width beyond which MEA 22 can span and support the pressure differential without deforming. Deforming of MEA 22 may result in a compromise of the structural integrity of cell system 20.
One manner of accommodating the presence of first gap 30 and the problems associated with pressure differentials involves incorporating a thin metal or polymer protector ring 32 into the electrochemical cell. Protector ring 32 supports the pressure load imposed on MEA 22 over first gap 30. At high cell operating pressures, however, internal cell dynamics associated with repeated pressure cycles may cause relative motion between cell components, which may dislocate protector ring 32 even after successful cell assembly and cause the presence of a second gap 34 of dimension l2 between protector ring 32 and cell frame 26. The dislocation of protector ring 32 may result in the exposure of MEA 22 to gaps 30, 34, which may cause less than optimum performance of the cell to be realized.
The maintaining of compression within the cell and the containment of the various electrochemical reactants and by-products generated in the cell is achieved by the use of thin, non-resilient gaskets, which are typically fabricated from polytetrafluoroethylene. When placed under the clamping loads encountered within the electrochemical cell, these non-resilient gaskets creep or deform to fill any imperfections in the surfaces of the components that they are intended to seal. The internal pressures that are effectively contained using such clamping methods may be considerably less than the pressure load exerted on the gaskets prior to any internal pressure being generated. As a result, the containment of high pressures using the non-resilient gasket approach requires very high clamping loads, which may, over the lifetime of the cell, become impractical. Furthermore, since such gaskets are non-resilient, they are ineffective at accommodating any creep that may occur as a result of a lessening of the clamping load. As such, they are likely to develop leaks over time as creep effects cause the clamping load to be relaxed. Moreover, the non-resilient gaskets may require a time consuming creep-inducing xe2x80x9cheat soakxe2x80x9d procedure to initiate the sealing of components.
While existing protector rings and gaskets are suitable for their intended purposes, there still remains a need for an improved apparatus and method of maintaining the compression of the cell and of protecting the MEA, particularly regarding the bridging of the gap between the flow field support member and the cell frame and the retaining of the protector ring thereacross during both assembly and operation of the cell. Therefore, a need exists for an integrally structured cell frame/flow field support member that allows cell compression to be maintained while protecting and supporting the MEA.
The above-described drawbacks and disadvantages are alleviated by an electrochemical cell system in which a cell frame is integrated with a flow field support member. The cell system includes an electrode, a proton exchange membrane and the flow field support member disposed at the electrode to support the electrode, the cell frame disposed at the flow field support member, and a membrane support element integrally formed with the flow field support member and the cell frame. The integration of the flow field support member and the cell frame defines a contiguous surface extending from and including the flow field support member and the cell frame. A resilient seal may also be disposed at the cell frame, the resilient seal being configured to inhibit fluid communication along an interface of the cell frame and the engaging surface of the cell component. A method of integrating the cell frame with the flow field support member includes disposing the membrane support element in a gap between the cell frame and the flow field support member and melting the membrane support element into the cell frame and the flow field support member to form a contiguous surface. A method of sealing a flow field of an electrochemical cell includes disposing a resilient seal at a cell frame of the electrochemical cell system.